Quoting the manpage:
int select(int nfds, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds, struct timeval *timeout);
nfds is the highest-numbered file descriptor in any of the three sets,
plus 1.
EBADF An invalid file descriptor was given in one of the sets. (Per‐
haps a file descriptor that was already closed, or one on which
an error has occurred.)
That's not quite how Linux behaves. We only check the fd_set up to the
maximum number of fds allocated to this task:
rcu_read_lock();
fdt = files_fdtable(current->files);
max_fds = fdt->max_fds;
rcu_read_unlock();
if (n > max_fds)
n = max_fds;
(then we copy in up to 'n' bits worth of bitmaps).
It is pretty straightforward to demonstrate that Linux doesn't check:
int main(void)
{
int ret;
struct timeval tv = { };
fd_set fds;
FD_ZERO(&fds);
FD_SETFD(FD_SETSIZE - 1, &fds);
ret = select(FD_SETSIZE, &fds, NULL, NULL, &tv);
assert(ret == -1 && errno == EBADF);
return 0;
}
Linux has behaved this way since 2.6.12, and I can't be bothered to get
out the historical git trees to find out what happened before 2005.
So ... if I change this behaviour by checking all the file descriptors, I
do stand a chance of breaking an application. On the other hand, that
application could already have been broken by the shell deciding to open
a really high file descriptor (I'm looking at you, bash), which the program
then inherits.
Worth fixing this bug? Worth documenting this bug, at least?
